1. Field of the Invention
Embodiments of the present invention relate to a pulse generator for generating pulses with a selectable variable width and/or delay. Such an arrangement may be used in any system where an accurately designed phase shift or variable pulse width or delay is required. For example, such an arrangement may be used for generating an accurate digitally controlled pulse in an optical disk writer, such as a DVD writer. Such an arrangement may also be used as part of a system for correcting quadrature error in a quadrature frequency changer of a radio frequency tuner. Thus, embodiments of the present invention also relate to an optical disk writer and a tuner including such a pulse generator.
2. Description of the Related Art
FIG. 1 of the accompanying drawings illustrates a conventional quadrature frequency converter. Such a frequency converter may be used, for example, in radio frequency tuners, and an example of an upconverter of this type is disclosed in British Patent Application No. 0511585.2. Such frequency changers may also be used in modulators where cancellation of a sideband is required.
The frequency changer comprises an in-phase (I) mixer 1 and a quadrature (Q) mixer 2, which are shown as receiving separate input signals but which may in other examples receive a common input signal. A local oscillator 3 supplies signals, which may be of fixed or variable frequency, to a quadrature splitter 4. The splitter 4 generates commutating signals which are in phase-quadrature and supplies these to the mixers 1 and 2. The outputs of the mixers 1 and 2 are supplied to a summer 5, which forms the vectorial sum of the output signals of the mixers. The resulting signal is amplified by an amplifier 6 and supplied to an output 7 of the frequency changer.
Where such a frequency changer is used in order to suppress or substantially cancel a sideband, for example including an image channel in a tuner, the quality of suppression depends on the quadrature phase balance between the commutating signals supplied by the splitter 4 and between the input signals supplied to the mixers 1 and 2. In the case of the commutating signals, known arrangements cannot achieve better than a 1-2° phase balance because of normal manufacturing spreads and tolerances that occur in practice. This therefore limits the suppression performance of such a frequency changer for normal applications, and special measures are necessary in order to improve the quadrature balance.
U.S. Patent Application Publication 2003/0071665 discloses a frequency multiplying arrangement using delay elements whose individual delays are non-controllable.
U.S. Pat. No. 4,922,141 and Japanese Patent No. 08274602 disclose phase-locking arrangements for delay lines which delay incoming signals. For example, U.S. Pat. No. 4,922,141 discloses a phase-locked loop delay line having two strings of delay elements, where the delay provided by each element is controllable. One string of elements is used to delay a data signal, whereas the other string is used inside a phase-locked loop arrangement for controlling the individual delay element delays. A delayed oscillator signal is compared with the non-delayed signal with the total delay being intended to be equal to one period of the oscillator signal. A charge pump is pumped up or down depending on the relative delay between the same edges of the direct and delayed signals, and the output of the charge pump varies the delay element delays so as to phase-lock the system. The delay provided by each delay element is thus fixed and determined so that the data signal to the independent string of delay elements can be delayed by a stable and well-defined amount.
Accordingly, what is needed is a pulse generator with improved width and/or delay control.